cannot in 'not only...but also' [duplicate]

I know from verious other discussions that 'can not' and 'cannot' are basically both correct but 'cannot' is being used more often. Nonetheless what happens if I have a 'not only...but also' construction like in the following sentence:

"Heat pumps cannot only be used for space heating but also for space cooling."

Is it also correct to use 'cannot' in this context? I'd appreciate every comment.

Solution 1:

You have two problems here. Let's start with correlative conjunctions and parallelism . . .

Correlative conjunctions—either/or, neither/nor, not only/but [also]—come in pairs. What follows each part of the pair must be parallel: both nouns, both verbs (same tense), both prepositional phrases, both independent clauses. In the case of your sentence, it should read:

Heat pumps can be used not only for space heating but also for space cooling.

or

Heat pumps can be used for not only space heating but also space cooling.

Now you have parallel prepositional phrases or parallel noun phrases. And, voila, the use of cannot is impossible.

But even if we used parallel verbs, cannot would still be incorrect:

*Heat pumps cannot only be used for space heating but also be used for space cooling. (incorrect)

You need to use can not. Why? Simply because not belongs to the correlative conjunction pair not only/but also—not to can:

Heat pumps can not only be used for space heating but also be used for space cooling.